I would be grateful if it continues to have monthly security updates already.

TBH, it's not even getting that!!
Last update...June...nothing for July or August to date...
Probably best to accept that there will be no more updates for the dteks, they are basically 'legacy' devices, and have been since they were 8-9 months old....

I think BlackBerry Dosent able to update dtek devices.
it behave with Customer like MI/redmi

There is no technological reason making it impossible for the Dtek series devices to be upgraded to Nougat (or Oreo, for that matter). Other devices running the same chipset, such as the Moto G4 Plus, are running Nougat now, and the Moto G4 Plus has been promised an eventual Oreo upgrade too.

It's purely a business decision.

Blackberry originally sold the devices under a "security" banner, and since then they've been fairly consistent with respect to delivering security patches for nearly 2 years, so I'd say they've delivered on their promise.

Nobody ever guaranteed any OS version upgrades at all, so Blackberry has been totally within its rights to have left the devices on Marshmallow. (Regardless of how understandably disappointed users might feel about that decision.)